UAVs have increasing application for defence and civil applications and are used for many purposes including surveillance, surveying, exploration and security.
Various designs of UAV are in current use. Some are of ducted fan type in which a rotary fan, propeller (or ‘prop’) assembly, driven by an engine, is enclosed within a shroud. Others are of fixed wing type or helicopter type with un-shrouded propeller or rotor, and still others are of hybrid type such as described in U.S. Pat. No. 6,270,038 assigned to Sikorsky Aircraft Corporation.
UAVs are constructed to be light and powerful for their size in order to give desired range, flight duration and air speed performance. The engine and its associated equipment are typically constructed of metal. Lightweight metals are preferred in order to reduce overall weight of the UAV and thereby achieve or maximise the aforementioned desired performance characteristics. Lightweight materials allow for more and/or improved noise reduction features/components on the UAV for a given total UAV mass, which help to make the UAV as quiet as possible. This is particularly beneficial for surveillance or security operations.
However, lightweight metals are either expensive, such as titanium, or have relatively low heat resistance characteristics, such as aluminium, and often require cooling mechanisms to prevent them melting. Aluminium is typically preferred because it is relatively cheap and capable of being moulded, cast or machined into suitable components. However, aluminium has a relatively low melting point. This is not a problem for most metal components on a UAV. But a specific problem arises when aluminium is used for or as part of the engine exhaust outlet, such as the exhaust manifold. Exhaust gases from the engine of a UAV under certain operating conditions can be sufficiently hot to melt an aluminium exhaust outlet.
This is a particular problem when demanding full power from the UAV, such as when full speed, increased climb rate or heavy payload lift is required. Ducted air cooling over the exhaust outlet can help to cool the outlet sufficiently for some engine operating conditions.
A known strategy for maintaining UAV exhaust temperature below a certain temperature limit is to run the engine air-fuel mixture richer than stoichiometric. Under such operating conditions the excess fuel helps to cool the exhaust gases. This strategy is often adopted partly because there are no emissions control regulations for UAV small engines.
However, using a rich air-fuel ratio uses excess fuel and does not generate more power; rather, it penalises fuel economy and thereby limits range and performance of the UAV. Rich air-fuel ratios also degrade engine stability through potential rich misfire which can occur.
On a UAV, stability is important due to vibration sensitivity of certain payloads, such a cameras mounted to the air frame. Improved fuel consumption would also give greater range or reduce the fuel payload to allow for higher airspeed or increased strategic payload (e.g. cameras, batteries or communications equipment).
It is important to note that miniature aircraft (such as model aircraft) engines typically run on a rich fuel-air mixture. This is primarily for engine durability and also often due to the lack of precise fuel-air ratio control on such small engines. Also, given the very small amounts of fuel burnt during a typical miniature aircraft flight (duration not being relatively very long in the air), fuel economy is of little importance. Hence running rich and the associated poor fuel economy is not perceived as a problem but a durability benefit. For these reasons, running a miniature engine lean would not typically be perceived as a preferred mode of operation for such engines.
One known prior art document, US 2003/0060962, directed to miniature aircraft discusses microprocessor controlled fuel-air ratios selected from a look-up table. However, other than suggesting selecting suitable fuel-air ratios automatically without controller (user) input, the document is silent about control of exhaust gas temperature to prevent damage to components, or even any control of exhaust gas temperature for any reason.
Attempts to run a leaner mixture in certain aircraft engines have focused on the need to target (reach) a peak exhaust gas temperature, typically around 620° C. to 720° C. For example, WO 2012/012511 discloses a fuelling strategy that specifically sets a target exhaust gas temperature, and modifies the air-fuel ratio, ignition timing and/or fuel injection timing to reach that target temperature. A feedback loop is used to help maintain the desired target temperature.
Fuelling strategies have been tried which adopt a closed loop system utilising feedback to control fuel-air mixture. Such closed loop strategies typically limit the lean burn mixture to prevent going over lean and to return the fuel-air ratio to a richer mixture when required.
One known prior art document, U.S. Pat. No. 7,658,184, focuses on the intersection between rich exhaust gas temperature signals and lean exhaust gas temperature signals to target a peak fuel-air ratio for the cylinder head assembly to maintain an optimal fuel economy. U.S. Pat. No. 7,658,184 does not however consider or discuss the problem of overheating lightweight exhaust components or a fuelling strategy to control exhaust gas temperature to prevent damage to such exhaust components.
It has been realised in the present invention that there are practical benefits in reducing peak exhaust gas temperature well below these limits in order to protect the integrity of lightweight aluminium based exhaust components. Keeping the peak exhaust to around 550° C. or less helps to achieve this goal
It is known that some such strategies in the prior art simply adjust the fuel-air ratio richer or leaner to maintain a desired emissions requirement or for fuel or engine efficiency, and mainly to compensate for altitude or atmospheric conditions, such as to control engine speed to maintain altitude or airspeed.
Many gas fuelled aircraft engines are known to operate in the region lean of a stoichiometric air fuel ratio, and in some cases tend to run lean overall. In such engines there is no need to operate with a richer than stoichiometric fuel-air ratio since the mixing of air and gaseous fuel is better compared to gasoline fuelled engines. Hence the benefits of richening in a gasoline engine for power and then component protection benefits are not so relevant. Accordingly, it would not be expected to control exhaust gas temperature by way of richening in such engines to prevent damage to the exhaust components.
For example, published patent application US 2009/0076709 discloses a strategy for controlling gas fuelled engines. The strategy attempts to control engine speed by varying the fuel-air ration in order to maintain a target engine speed. Exhaust gas temperature is used as an input to try to maintain the required engine speed. This document teaches the opposite of the present invention. This document does not envisage controlling fuel-air mixture in order to manage exhaust gas temperature in order to protect exhaust components from melting.
None of the aforementioned known fuelling strategies control exhaust gas temperature to protect the exhaust system components.
The present invention has been conceived in light of the aforementioned problems. It has been found desirable to provide improved combustion management in a UAV engine to control exhaust gas temperature and thereby help preserve the exhaust gas outlet from heat damage and to maintain engine stability.
Furthermore, it has been found desirable to limit exhaust gas temperature to a specific threshold value to protect the integrity of the exhaust gas system components, and particularly to prevent higher exhaust gas temperatures being reached which may otherwise melt the exhaust components.